Electrostatic copyholder and method for making same



June 3, 1969 A- Q. TESTONE ELECTROSTATIC COPYHOLDER AND METHOD FOR MAKING SAME Filed May 24, 1966 6 37 one- (AWE/V701? l2 ANTHO/VV'O. TESTOIVE ATTORNEYS.

United States Patent 3,448,356 ELECTROSTATIC COPYHOLDER AND METHOD FOR MAKING SAME Anthony Q. Testone, Lansdale, Pa., assignor to The Simco Company, Inc., Lansdale, Pa., a corporation of Pennsylvania Filed May 24, 1966, Ser. No. 552,483 Int. Cl. G03b 15/00; H05f 7/00 US. Cl. 317-262 14 Claims ABSTRACT OF THE DISCLOSURE A copyholder to which articles are adhered by electrostatic forces. A first conductive member overlies one surface of a dielectric panel and a second conductive member constituting a grid with open areas overlies the second surface. A D-.C. high voltage coupled across the two conductive members enables conductive articles to adhere to the second surface by virtue of the electrical field between the conductive members and emanating through the open areas of the grid. Non-conductive articles adhere by virtue of charges induced therein which are opposite in polarity to those produced on the second surface by the DC high voltage.

This invention relates to an improved electrostatic copyboard for retaining articles in position by electrostatic force preparatory to photographing or other operations, and more particularly relates to a method for fabricating such a copyboard.

In prior U.S. Patent No. 3,359,469 entitled, Electrostatic Pinning Method and copyboard, there was shown a fiat block or frame to the surface of which articles may be placed and then secured in position by electrostatic for'ces. The invisible electrostatic attractive forces eliminated the need for adhesives, tacks, pins, overlays and/or vacuum hold down constructions, all having obvious disadvantages especially well known to those skilled in the photographic and copying arts. In the foregoing patent application, it was demonstrated that the repetitive manipulation of non-conductive sheets during successive positioning could and would cause degeneration of the electrostatic pinning force with respect to a particular charge applied to the copyboard surface. However, it was additionally shown that such a diminuition of attractive force could be countered and immediately re-established to original intensity simply by reversing the polarity of voltage applied to the board. Nevertheless, the prior construction was somewhat inefficient in its use in connection with conductive articles and objects. Such was also true with non-conductive articles as well, for example atmospheric moisture conditions (high relative humidity) and even operation personnel perspiration conditions, caused a film of electrolyte to be deposited on the objects surface so as to impart a conductive effect thereto. That is, the principles of electrostatic attraction are governed by Ooulombs Inverse Square Law-the force between two electrostatically charged bodies is proportional to the product of the magnitude of the charges on the bodies and inversely proportional to the square of the distance between them-expressed mathe matically:

where I F is the force of electrostatic attraction (or repulsion) between the charged bodies;

Patented June 3, 1969 It has been theorized in the art that the collection of the relatively high conductivity water fi-lrn at the interface between the two charged bodies reduces the etfective dielectric constant K. Hence, other investigators have endeavored to reconstitute a high electrical resistance at the barrier interface by evaporation of the water film and also providing a semi-conductive film or laminate at the interface, for example, one having a resistivity in the range 10 to 10 ohm centimeters. However, it has been found that while such semi-conductive laminates may improve the electrostatic attractive force upon conductive objects there was an accompanying reduction and in fact corresponding deterioration, of the attractive forces upon non-conductive objects.

The present invention relies on the principles of Coulombs Inverse Square Law in its application to nonconductive objects and, in addition, upon the fact that, within its shape limitations, a conductive body (or one rendered conductive by virtue of a Water film) tends to orient itself parallel to the lines of force of an electric field. The conductive body endeavors also to move perpendicular to and in the direction of the most concentrated or densest electrical field. Accordingly, the electrostatic holding device of the instant invention depends upon the creation of a plurality of charged condensers whose plates are so oriented that the lines of force of their electrical field are densest just below the surface of the board and in a direction parallel thereto while at the same time projecting a plurality of arcuate field patterns above the surface. Furthermore, the surface of the board is also fabricated to have an optimum effective area of non-conductive construction upon which electrostatic charges of a given polarity may be imposed for attracting and efiiciently retaining non-conductive articles and sheets.

It is therefore an object of this invention to provide a new and improved electrostatic copyboard in which there is an enhancement of retaining forces for conductive articles, and those rendered conductive by virtue of a water film, without degrading the holding forces for non-conductive articles.

Another object of this invention is to provide an electrostatic copyholder in which a plurality of discrete electric field mosaics are formed by a grid construction defining a multiplicity of condensers within the surface of the copyholder while at the same time establishing a dielectric surface upon which electrical charges may be imposed.

Still another object of this invention is to provide a method for making an electrostatic copyboard which will accomplish the condenser-charged surface configuration by a painted-on grid arrangement.

Yet another object of this invention is to provide a method for making a grid-type electrostatic copyholder utilizing techniques which are simply effectuated.

Other objects of this invention are to provide an improved device and method of the character described that is easily and economically produced, which is sturdy in construction and both highly efiicient and effective in operation.

With the above and related objects in view, this invention consists of the details of construction and combination of parts as will be more fully understood from the following detailed description when read in conjunction with the accompanying drawing, in which:

FIGURE 1 is a perspective view of a new and improved electrostatic copyboard embodying this invention.

FIGURE 2 is a sectional view taken along lines 22 of FIGURE 1.

FIGURE 3 is a sectional view taken along lines 3-3 of FIGURE 2.

FIGURE 4 is a sectional view taken along lines 4-4 of FIGURE 3.

FIGURE 5 is a perspective view of a conductive grid configuration applied to the rear surface of dielectric panel embodied in this invention.

FIGURE 6 is a perspective view of a conductive grid configuration applied to the front surface of the dielectric panel.

FIGURE 7 is a fragmentry perspective view of the copyboard showing the electric field of force in broken lines.

Referring now in greater detail to the drawing in which similar reference characters refer to similar parts, there is shown an electrostatic copyholder, generally designated as A, to the surface to which a sheet or sheets of design material, generally designated as B, are detachably secured by an electrostatic pinning force preparatory to photographing or other composition work.

The copyholder A comprises a stiff flat insulative base plate 12 to whose front face is adhesively secured a grid panel C. The base plate 12 may be made of wood but is preferably fabricated of a pressed fiber-board wood composition, such as tempered masonite, which is plastic coated on both sides for waterproofing and anti-warping. The base board 12 may be /2 inch thick for suitable subjacent support and can have dimensions which are rectangular and/or suitable for the needs of a particular customer.

Interposed between the grid panel C and the base board 12 is a dielectric panel 14 which affirmatively insulates the hereinafter discussed conductive back surface of the panel C from the base plate. The insulating panel 14 is preferably a polyester film, such as Mylar, and IO-mil (.010 inch) Mylar has been determined to have excellent dielectric characteristics for this purpose. The film panel 14 is secured to the surface of board 12 with a doublebacked adhesive tape interfacial arrangement at 13, such as Permacel #02 tape, which is a 2 mil rope-paper backing coated on both sides with a solvent resistant adhesive. Similarly, the grid panel C is adhesively secured to the insulating film 14 by a second layer 15 of doublefaced adhesive tape.

The grid panel C is also fabricated of l0-mil Mylar polyester film and has identical dimensional configuration as both the backboard 12 and the film 14. To the hack face of the panel C is applied a conductive pattern 20, which, in the case of a rectangular board A, has the shape of a rectangular border 18 enclosing a plurality of diagonally arranged stripes 22 therein. Refer to FIGURE 5. The border 18 is spaced from the periphery of the panel C by a clear margin 19 approximately 1 /2 inches in width since high voltage will he applied to pattern 20. The border 18 may be one inch wide all the way around, and the stripes 22 themselves may be /2 inch wide and spaced uniformly from each other by clear zones or nonpainted stripes 21. The entire pattern .20 may be incorporated by using a stencil and painting or rolling a conductive coating, such as Butens No. 13976 conductive paint, through the stencil. It is important to note that the stripes 21-22 on the back surface are arranged diagonally. However, while the configuration shown is at a 45 angle from the base, other inclinations varying from 30 to 60 from the base may also be satisfactory. In this regard, the angular disposition of the stripes 22 is only significant in its relation to the configuration of the stripes 32 on conductive pattern 30 on the opposite face of the grid panel C.

The conductive grid pattern 30 on the front face of panel C is also a geometric configuration and includes an identical conductive coating such as that on the rear surface. See FIGURE 6. In this case, however, the ground side of a DC. high voltage generator source G is applied so that the border 28 may extend directly to the periphery of the panel. The conductive stripes or bands 32 are disposed vertically with alternate longitudinally extending clear areas 31 intermediate adjacent bands. The border 28 is approximately one inch in width about the periphery, and the stripes 32 and clear zone 31 may each be /2-inch Wide. It is Worth mentioning again that the orientation of the stripes 32 with respect to the stripes '22 may be reversed so that the front pattern 30 may have diagonal bands whereas the rear pattern could have vertically or horizontally-disposed bands.

Referring now to FIGURES 2, 3 and 4, a metal flathead screw 24- is mounted in the lower right hand corner of the base board 12 and has a head which is in abutment with and directly connected to the border 18 of the high voltage grid pattern 20. A blob of conductive cement 2-5 is used to adhesively secure the screwhead 24 in firm electrical contact with the high voltage pattern 20. A terminal 26 is coupled to the rear portion of the screw 24 and extends downwardly within a bore 27 in the base plate 12. A male connector 34 is formed on the end of wire conductor .35 running through shielded cable D and is received within the bore 27 for engaging the female terminal 26. The lead 35 is connected to the high voltage side of the DC. generator G. The low voltage or ground side of the DC. generator G is connected to the shielding braid 37 which surrounds the insulation of cable D.

As is best illustrated in FIGURE 4, the grounded shielding braid 37 is connected to binding post portion 38 at the rear of flathead screw 40. The shank of the screw 40 passes through the clear margin 19 about pattern 20, and the underside of the screwhead 40 cemented at 39 With a blob of conductive adhesive to the border 28 of pattern 30. As is apparent, the patterns 20 and 30' are spaced from each other by dielectric film C of Mylar so that when the DC. high voltage is applied thereacross, the

patterns act as the opposed plates of a condenser. However, the stripes 22 are oriented obliquely with respect to the stripes 32. As a consequence, while the greatest concentration of the electrical field will be in the area where the stripes 32 overlie the stripes 22, the edge lines of force will pass through the clear zones 21 and 31 to define an electrical field which extends arcuately beyond the surface. See FIGURE 7. It is on the edgefield phenomenon with which this invention is specifically concerned.

Referring now back to FIGURE 2, the entire front face of the panel C, the peripheral edges of the base plate 12 and its rear marginal surface is then encapsulated with a polyvinyl fluoride shrink film 42, such as Tedlar. The shrink film 42 thus seals the edges of the board A and protects the exterior conductive pattern 30 by covering it. Thereafter, the edges of the board A are framed with a bezel 44 made of extruded polyvinyl chloride moldings which are mitered together and secured to the 'base plate 12 with finishing screws 46. Finally, a cover sheet 48 is hung from the upper edge of the copyboard A across the face thereof and acts as the active working surface. The cover sheet 48 is insulative in character and may be a fiber glass calendered sheet which is impregnated with silicon rubber.

The high voltage generator G is a conventional D.C. electrostatic power source which can develop a DC. potential in the range 5,00015,000 volts. The generator G is shown in block form and for purposes of the present invention develops a positive output when switch 50 is closed. Accordingly, a positive voltage will be imposed on the grid pattern 20 whereas the grid pattern 30 wil1 be at ground or negative. In this case, abutting a nonconductive sheet B against the face of the copyboard A, i.e. on cover shade 48, will cause the non-conductive sheet B to stick with a fair degree of attraction by virtue of polarization. However, if the sheet B is maintained in abutment for a short period of time and/or rubbed while abutting by a grounded operator, the sheet will pick up negative charges from ground. The net negative charge on the nonconductive sheet will cause it to be strongly attracted toward the positively charged grid pattern 20, especially Where the clear areas 31 overlie a conductive stripe 22. However, if, in addition, the sheet B were rendered conductive by a water film or if it were already a conductor as in the case of a metal foil, or even very slightly conductive, as for example, paper with about 5% moisture content, it would now be drawn by the arcuate fringe field or edge leakage lines of force which would extend from the stripes 22 through the clear areas 21 and 31 and back in an arcuate pattern to the stripes 32. These lines of force are demonstrated in broken lines in FIGURE 7. The field is most dense where the plates or stripes 22 and 32 overlie one another. However, these fields of maximum density do not penetrate beyond the overlying ground pattern, and are therefore not effective in attracting conductive objects. Nevertheless, the edges of the fields of the multiplicity of small condensers are effective to form fringe lines of force which, though of a lesser intensity, do extend beyond the surface of the cover sheet 48 and are most efiicient in drawing down conductive or even slightly conductive objects toward the field of maximum concentration, and pulling them down from a short distance.

The present copyboard, as did that of prior Patent No. 3,359,469, also permits a renewal of the charge by switching of the power supply G. The use of the switching arrangement (reversal of the polarity of voltage applied to the board) to accommodate diminution of attractive force is of even more importance with regard to the securing of conductive sheets B. That is, since thesheets B are, by definition, conductive, the surface charge is drawn oif the board much more rapidly than is the case of non-conductive sheets. Accordingly, switching must be performed between each successive application of conductive sheets rather than after a plurality of applications of non-conductive sheets which causes attrition.

For example, if the grid 22 first has a high positive voltage while the grid 32 is at ground, the arcuate field pattern would draw down the conductive sheet B. However, where the sheet B is further rubbed by the grounded operators hand, the sheet would immediately acquire a negative charge (from ground). When the conductive sheet is removed from the board, its negative charge is partially left behind upon the surface of the board to counterbalance the positive polarized charge on the boards surface. As a consequence, the whole surface of the board is effectively at ground potential and hence inoperative.

Should the voltage from the power supply G now be switched, the grid 22 would be at ground potential. Note also that the grid 32 would remain at ground potential since it is permanently connected to ground. However, the clear zones and spaces 21 and/or 31, where the two grids 22 and 32 do not superimpose, immediately drop to a negative potential. Therefore, arcuate lines of the field will emanate from these negative areas and terminate at the grounded stripes 22 and 32. Accordingly, the board is once more prepared to attract conductive sheets by virtue of the alternately established arcuate or secondary field. The board A is also prepared to hold non-conductive materials because of the negative voltage on the parts mentioned and act in the manner described in the prior application, Ser. No. 362,049.

In passing and in summary, it is to be observed that the diagonal orientation of one set of stripes 22 with the other set 32 reduces the likelihood of overlapping condenser plates and enables a staggering of the areas through which the fringe or leakage field may extend.

Still another mode of securing a maximum staggering of the condenser plates would be by a pair of checkerboard patterns whose conductive squares are arranged to overlie clear squares in the superimposed pattern.

Although this invention has been described in considerable detail, such description is intended as being illustrative rather than limiting, since the invention may be variously embodied, and the scope of the invention is to be determined as claimed.

What is claimed is:

1. An electrostatic copyholder comprising a rigid insulative board, a dielectric panel secured to one face of said board, a first conductive grid intermediate the board and said panel, a second conductive grid on the opposite face of said dielectric panel and having a pattern of spaced apart conductive elements overlying said first grid and insulated therefrom, an insulative cover secured over the surface of said second conductive grid, and means for connecting a DO. high voltage across said first and second grids, said first and second grids being so constructed and arranged with respect to each other that the electrical field between the two grids will emanate edge fringe lines passing beyond said cover in an arcuate spray, whereby nonconductive articles placed in abutment with said cover will electrostatically adhere thereto by virtue of charges induced in said articles opposite in polarity to those on said cover produced by said high voltage means, and conductive articles juxtaposed with said cover will adhere thereto by virtue of the edge fringe lines of said field.

2. The invention of claim 1 wherein said first and second grids each comprise a plurality of stripes and the stripes of the first grid being obliquely oriented with respect to the stripes on said second grid.

3. The invention of claim 2 wherein the stripes of said first grid are diagonally disposed with respect to the stripes of said second grid.

4. The invention of claim 3 wherein the stripes of said first grid are enclosed within a conductive border peripherally spaced from the margins of said panel, and the stripes of said second grid are enclosed within a conductive border at the periphery of said panel, and the high side of said high voltage means being connected to said first grid.

5. The invention of claim 4 wherein each of said grids comprises a painted-on conductive coating.

6. The invention of claim 5 including an insulative film intermediate said board and said first grid.

7. The invention of claim 6 wherein said cover comprises a polyvinyl shrink film.

8. The invention of claim 4 including means to cut oif the supply of high voltage to said first grid and at the same time grounding said first grid.

9. The invention of claim 8 wherein the means for connecting the high voltage across said first and second grids comprises a terminal having a flat head connected to the border of said first grid, and a second terminal passing through the marginal portion about the border of said first grid and having a fiat head connected to the border of said second grid.

10. An electrostatic copyholder comprising a rigid insulative board, a dielectric panel secured to one face of said board, a first conductive member intermediate the board and said panel, a second conductive member constituting a grid on the opposite face of said dielectric panel and having a pattern of spaced apart conductive elements with interstices there'between, an insulative cover secured over the surface of said second conductive member, and means for connecting a DC. high voltage across said first and second conductive members, said first and second conductive members being so constructed and arranged that the electrical field created therebetween will emanate edge fringe lines passing beyond said cover in an arcuate spray, whereby non-conductive articles placed in abutment with said cover will electrostatically adhere thereto by virtue of charges induced in said articles opposite in polarity to those on said cover produced by said high voltage means, and conductive articles juxtaposed with said cover will adhere thereto by virtue of the edge fringe lines from said field passing through the interstices of said grid.

11. The invention of claim 10 wherein said first conductive member constitutes a grid having a pattern of spaced apart conductive elements and said first and second grids each comprises a plurality of stripes with the stripes of said first grid being obliquely oriented with respect to the stripes on said second grid.

12. An electrostatic copyholder comprising a dielectric panel, a first conductive member overlying one face of said dielectric panel, a second conductive member constituting a grid overlying the second face of said dielectric panel and having spaced apart open areas between conductive portions, and means for connecting a DC. high voltage across said first and second members, said first and second conductive members being so constructed and arranged that the electrical field created therebetween will emanate edge fringe lines passing through the open areas and beyond the grid in an arcuate spray, whereby conductive articles juxtaposed upon the second face will adhere thereto by virtue of the electrical field between said conductive members emanating through the open areas of said second conductive member, and non-conductive articles will adhere to the second face by virtue of charges induced in said non-conductive articles opposite in polarity to those of said second face produced by said D.C. high voltage.

13. The invention of claim 12 including an insulative sheet covering said second conductive member.

,14. An electrostatic copyboard comprising a first conductive member, a second conductive member constituting a grid having spaced apart open areas between conductive portions, a dielectric panel interposed between said first and second conductive members, and means for connecting a DC high voltage across said first and second conductive members, said first and second conductive members being so constructed and arranged with respect References Cited UNITED STATES PATENTS 3,194,131 7/1965 Robinson. 3,337,784 8/1967 Lueder 317-262 LEE T. HIX, Primary Examiner.

US. Cl. X.R. 

